Fair organizer Sanford Smith’s "Modernism: A Century of Style and Design" ﬁrst opened at the Park Avenue Armory in the fall of 1986. Back then, it was a pioneer and spawned shows of "modernism" -- 20th-century decorative arts -- across the country, and even internationally. Now, it’s joined at the hip to Art20, another of Smith’s dec arts fairs. Both opened this weekend.